IGBTs Transmit Woe to Analog Signal Conditioning

The ship date is looming, but a noisy VFAC drive creating huge signal spikes refuses to be calmed.

Back in the mid to late '90s, I worked for a small engineering firm that makes automated testing equipment for the fluid power industry; test stands for things like hydraulic pumps, motors and valves. In combination pump/motor test stands, we typically used Variable Frequency AC drives for the prime mover and loading device. On one particular machine, we selected a new and new-to-us VFAC drive based on its price, small size and advanced capabilities. This one was rated for 100 HP (74 kW) and ran on 480 VAC, three phase power.

This drive was working out nicely, but for one BIG problem, noise. Our single-ended (long story) analog signal conditioning for flows, pressures and torque were picking up noise in a big way. Intermittent, irregular and relatively infrequent, HUGE spikes were swinging the proper signal either high or low by as much as 25% full scale. It didn't take long to realize that the VFAC drive was the source. Stop the drive and everything became peaceful again.

Though I studied EE some at school, I'm primarily a software guy. I tried ferrite chokes, filters, and variations on signal cable and shield termination. Nothing seemed to help much and I had pretty much used up my knowledge on the topic. Troubleshooting this problem quickly became a collaborative effort between me, the technicians I worked with, the North American drive reseller and the Japanese drive manufacturer. By this time, the test stand was late and our customer was in a bind. They already had an assembly line piling up parts that, by their certified ISO process, had to be tested before they could be sold.

The reseller sent their resident EE to our plant to have a look. Our effort at chokes and such weren't working because the very air was filled with high frequency pulses. One could hold a scope probe in the air and see a classic ringing waveform on the scale of tens of nanoseconds. Every wire that wasn't transmitting this was sure as heck receiving it. The drive OEM finally sent their senior design engineer from Japan to our little factory here in Nowhere, Oklahoma.

After much probing and brow-wiping, we were told that the IGBTs used in this drive were 5th generation, the newest at the time, and offered great benefits from the efficiency gained by their high switching speed. Well at 600 Volts on the DC buss, there's a lot of potential being modulated at a very high speed. Every time the IGBTs fired (about 14,000 times per second per IGBT), the airwaves lit up. We only saw irregular and infrequent spikes in our analog signals because our sample rate was so low and the duration of the pulses so short. But when we did see a spike, it swamped our signal.

A few tweaks were made to the drive's internal settings, to lower the modulation frequency and stretch the switching time a little. We even added a filter between the drive output and the motor leads. In the end, however, optical isolation was the real winner. Every interface wire that connected to the drive was acting as an antenna to transmit that noise throughout the entire test stand. Full credit goes to the reseller's EE who tried optically isolating the digital interfaces to the drive and the result was dramatic; we had a solution. We wound up designing an optical isolator circuit board of our own that handled all digital and analog interfaces to the drive, which we used in all relevant machine designs going forward.

Reagan Thomas studied EE and EET in school, but wound up becoming a software guy who dabbles with electrons. He worked 18 years designing software (and some hardware) for automated testing equipment. He now deals with software for radar video distribution equipment.

Good Story of Success, this is the problem virtually every design engineer faces with in his life, and everyone finds some special solution, but unfortunately there is no standard defined solution to this problem, thats Electronics. That is the reason we love electronics.

Whatever works. Reagan tried a number of valid hardware solution tests. With the cost involved and bosses no doubt breathing down necks, one goes with the first solution that solves the problem.
Good story, Reagan.

Isolation is indeed what it takes. I wound up using Analog Devices isolation amplifier modules back in the late 1970s for the four-quadrant drive used in engine test stands. The only difference is that I never even tried to go directly with the control and data wires. Isolation amps are sometimes woth their weight in gold. Thanks, AD.

This story exemplifies why the 4-20 mA current loop analog interface was designed. A current loop interface gives you optical isolation combined with a low impedance, differential input for maximum noise immunity.
If you are ever faced with this sort of problem again, you can buy 4-20 mA converters from vendors like Phoenix contacts. They take a 0-10V single ended input and convert it to an isolated 4-20mA current loop. To convert a current loop back to single ended voltage at the receiver end only takes a single resistor.
Another advantage to a 4-20 mA current loop is you can tell the difference between a zero signal and a cut cable.
http://www.phoenixcontact.com/signal-level-matching/31244_14827.htm#30531

The analog 4-20mA current loop does not make optical isolation easier.
It removes the necessity of taking the amplifier ground out to the field.
It was invented before optoisolators and the current range was higher.
You are confusing it with the digital adaptation, which switches between 4 and 20mA.

I suspect a poor layout, ground loops, poor or ineffective grounding, separation of critical circuits or components, etc. Using high speed IBGTs magnified the problem due the t6he faster switching.
The use of opto isolators solved the problem by removing or reducing the effects of parasitic "antennas" caused by an apparent poor layout.
Re-engineering the IGBT drive circuitry using EMC reduction techniques,is the better solution.

The problem was shoot through.
Definition of an H bridge
short waiting to happen.
IGBT's have nasty current tails that do not switch off even when the drive gate is totally discharged do to charge acumulation transistor decay times.
Speed up the IGBT turn on...see definition above
1) air core in upper drain
2) more dead time (less dead time dead IGBT's) (not a problem exept in high accuracy aplications)
3)1st and 2nd reduce source problem another solution (sometimes you need them all!) is to use the same clock or sense the power suply clock and interleave sampling so to not be on when H bridge switching.
4)LVDS
5) guard rings and ground planes around dif (or single ugg) signal.
6)Use that extra op amp to pic up RF, amplify, condition it and subtract it from signal in real time.
7) Two or more of above